case NaiveDateTime.from_iso8601(valid_time) do
      {:ok, datetime} ->
        case Somefunction(which give me true or false) do
          true -> true
          false -> false
        end

      {:error, _} ->
        "datetime is not valid"
    end

case NaiveDateTime.from_iso8601(valid_time) do
  {:ok, datetime} ->
    Somefunction(which give me true or false)

  {:error, _} ->
    "datetime is not valid"
end

valid_time
|> NaiveDateTime.from_iso8601()
|> do_validate_instead_of_case()

[...]

defp do_validate_instead_of_case({:ok, datetime}) do
  case Somefunction(which give me true or false) do
    ...
  end
end

defp do_validate_instead_of_case({:error, _}),
  do: "datetime is not valid"